add get device count to each backend for example
add get device count to each backend for example

tedsta/arrayfire-rust,arrayfire/arrayfire-rust,arrayfire/arrayfire_rust,9prady9/arrayfire-rust,arrayfire/arrayfire-rust,botev/arrayfire-rust,9prady9/arrayfire-rust,arrayfire/arrayfire-rust,9prady9/arrayfire-rust,jramapuram/arrayfire_rust,9prady9/arrayfire-rust